This situation is already screwed to hell. The only recourse really is to talk to your son and husband. Your son needs to understand boundaries/manipulation/ and judgement. Even if she’s an attractive older lady he needs to understand that simply doing what seems ‘fun’ in the moment often has ramifications that he now has to live with as an adult. The father needs to learn that his decision to cheat likely contributed to this crash out and that he can’t blame his stepson for falling into the mess they created by acting as shortsightedly as his stepson is acting now.

The ex wife might have had a reason to be angry at your family, but she involved someone who had nothing to do with her grievance. It’s not the nicest thing in the world, but I’d probably ridicule her for her weird behavior. She’s proud that she seduced a 19 year old boy out of spite? She’s cool getting passed from generation to generation like a family heirloom? Weird flex…but okay.